1. Field of the Invention
The present invention relates to a filter, and, in particular to a filter that is effective for realizing broadening of a pass band while satisfying requirements for a small size and a low loss.
2. Description of the Related Art
A band-pass filter for attenuating unnecessary components such as higher harmonics is used in a high-frequency circuit unit of a radio communication apparatus or the like. As the band-pass filter of this type, a filter using a dielectric resonator having a compact structure, with which a satisfactory damping property is obtained, is mainly used. In particular, a distributed constant type filter, which utilizes a strip line as a resonator formed in a dielectric, is widely used.
As the distributed constant type filter of this type, for example, a method described in JP-A-11-17405 is known. JP-A-11-17405 discloses a method of constituting a band-pass filter or a band elimination filter by coupling a λ/4 strip line and a λ/2 strip line.
In recent years, as in an Ultra Wide Band (UWB) system, a filter is required to broaden a band such that the band has a fractional band width (a pass band width/a center frequency) close to 100% or a higher fractional band width.
However, in the conventional designing method for a band-pass filter and the method disclosed in JP-A-11-17405, it is necessary to form a large number of resonators in multiple stages in order to realize broadening of a pass band. Increasing the number of stages of resonators in this way involves an increase in a size of a filter and an increase in an insertion loss.
In particular, in realizing an ultra-wide-band such as the UWB system, a necessary band is not obtained by two to three stages of resonators. Thus, it is difficult to provide a small and low-loss filter with the conventional method.